Process for size classifying ammonium sulfate crystals which are present in a suspension

ABSTRACT

The invention relates to a process for size classifying ammonium sulfate crystals using a screen, said process comprising feeding a feed suspension to the screen, said feed suspension comprising said ammonium sulfate crystals in an ammonium sulfate solution, size classifying the ammonium sulfate crystals, and keeping both sides of the screen immersed in liquid during said size classifying.

CROSS REFERENCE TO RELATED APPLICATION

This application is the National Phase of International ApplicationPCT/NLO02/00222filed Apr. 5, 2002 which designated the U.S., and thatInternational Application was published under PCT Article 21(2) inEnglish.

The invention relates to a process for size classifying ammonium sulfatecrystals using a screen, said process comprising feeding a feedsuspension to the screen, said feed suspension comprising said ammoniumsulfate crystals in an ammonium sulfate solution, and size classifyingthe ammonium sulfate crystals.

Processes for size classifying ammonium sulfate crystals are describedin JP-A-3150217 and in JP-A-426512. In the known processes a suspensionwhich originates from a crystallizer and which comprises ammoniumsulfate solution and ammonium sulfate crystals is supplied to a screen.Using the screen the suspension is separated in a coarse crystalfraction and into a fine crystal fraction. The fine crystal fraction isrecycled to the crystallizer, the coarse crystal fraction beingsubjected to drying to obtain ammonium sulfate product crystals.

The known processes have the disadvantage that they are sensitive toclogging of the openings of the screen by ammonium sulfate crystals,which results in a less effective separation.

The goal of the invention is to provide a process in which the cloggingof openings is prevented or at least reduced to a considerable extent.

This goal is achieved according to the invention by keeping both sidesof the screen immersed in liquid during said size classifying.

According to the invention a feed suspension comprising the ammoniumsulfate crystals to be size classified, is fed to the screen. The sizeclassifying according to the invention results in a permeate suspensionand a product suspension which may be withdrawn from the screen. Thepermeate suspension comprises ammonium sulfate crystals which have beentransported through the openings of the screen, and ammonium sulfatesolution which has been transported through the openings of the screen.The product suspension comprises ammonium sulfate crystals which havenot been transported through the openings of the screen and ammoniumsulfate solution which has not been transported through the openings ofthe screen.

According to the invention both sides of the screen are kept immersed inliquid. As used herein, keeping both sides of the screen immersed inliquid is intended to mean that the side of the screen to which the feedsuspension is fed, as well as the side of the screen from which thepermeate suspension is withdrawn, are kept immersed in liquid. As aresult of said immersing contact of the screen with air, and inparticular the openings of the screen with air is prevented. Withoutwishing to be bound by any scientific theory it is believed that thisprevents or at least reduces the occurrence of crystallization ofammonium sulfate from the ammonium sulfate solution, and as a resultreduces clogging of the openings of the screen. The liquid in which bothsides of the screen are kept immersed is preferably an ammonium sulfatesolution and/or a suspension comprising ammonium sulfate crystals in anammonium sulfate solution.

Both sides of the screen may be kept immersed in liquid by any suitablemethod, preferably by choosing and/or controlling the flow rate of thefeed suspension, the flow rate of the permeate suspension and/or theflow rate of product suspension relative to each other, such as to keepboth sides of the screen immersed in liquid. This may be done by anysuitable method, for instance by using inlets and outlets for having theappropriate dimensions, by using overflows or by using one or moreadjustable valves.

Preferably, use is made of a screening apparatus comprising a firstchamber, a second chamber and the screen, the screen forming a partitionbetween the first chamber and the second chamber, wherein the processcomprises feeding feed suspension into the first chamber, withdrawingthe permeate suspension from the second chamber, and withdrawing theproduct suspension from the first chamber. When using such screeningapparatus both sides of the screen can be immersed in liquid in aneffective way. The screen may separate the first chamber and the secondchamber in any suitable way. The screening apparatus may comprise ahousing, the screen dividing the housing into the first chamber and thesecond chamber. The apparatus may also comprise an inner vessel, forinstance a tube, the wall of said inner vessel including the screen, andan outer vessel, wherein the part of the wall of the inner vesselcomprising the screen is surrounded by the outer vessel. Preferably, oneend of the inner vessel, in particular one end of the tube, extendsthrough a wall of the outer vessel.

Preferably, the feed suspension being fed to the screen comprises lessthan 50 vol. %, more preferably less than 40 vol. %, in particular lessthan 30 vol. %, more in particular less than 25 vol. % ammonium sulfatecrystals, relative to the volume of the feed suspension. When theabovementioned screening apparatus is used, the feed suspension beingfed into the first chamber preferably comprises less than 50 vol. %,more preferably less than 40 vol. %, in particular less than 30 vol. %,more in particular less than 25 vol. % ammonium sulfate crystals,relative to the volume of the feed suspension. Decreasing the percentageof crystals in the feed suspension has the advantage that transport isfacilitated and that a higher percentage of fine crystals may beseparated without the screen running dry. There is no specific lowerlimit for the percentage crystals in the feed suspension. Generally, thepercentage of crystals in the feed suspension fed to the screen ishigher than 0.1 vol. %, preferably higher than 0.5 vol. %, morepreferably higher than 1 vol. %, in particular higher than 2 vol. %,relative to the volume of the feed suspension.

Preferably, the product suspension being withdrawn from the screencomprises less than 70 vol. %, more preferably less than 60 vol. %, inparticular less than 50 vol. %, more in particular less than 40 vol. %ammonium sulfate crystals, relative to the volume of the productsuspension. When the abovementioned screening apparatus is used, theproduct suspension withdrawn from the second chamber preferablycomprises less than 70 vol. %, more preferably less than 60 vol. %, inparticular less than 50 vol. %, more in particular less than 40 vol. %ammonium sulfate crystals, relative to the volume of the productsuspension. Decreasing the percentage of crystals in the productsuspension has the advantage that transport of the product suspension isfacilitated.

The ammonium sulfate concentration in the aqueous ammonium sulfatesolution is not limited to a specific value. Generally, the ammoniumsulfate solution contains at least 1 wt. % of dissolved ammoniumsulfate, preferably at least 5 wt. %, more preferably at least 10 wt. %,in particular at least 20 wt. %, more in particular at least 30 wt. %.,relative to the weight of the ammonium sulfate solution. Generally, theammonium sulfate concentration is lower than 60 wt. %, preferably lowerthan 50 wt. %, more preferably lower than 45 wt. %, relatave to theweight of the ammonium sulfate solution.

Preferably, transport of the suspension at the side of the screen towhich the feed suspension is fed takes place in a direction essentiallyparallel to the screen. When the abovementioned screening apparatus isused, transport of the suspension in the first chamber preferably takesplace in a direction essentially parallel to the screen. This has theadvantage that blocking of the openings by ammonium sulfate crystals isfurther reduced. Preferably, transport of the suspension at the side ofthe screen to which the feed suspension is fed (when the abovementionedscreening apparatus is used, in the first chamber) takes place at a rateof at least 0.01 m/s in a direction parallel to the screen, morepreferably at least 0.05 m/s, in particular at least 0.1 m/s, more inparticular at least 0.25 m/s. Increasing the flow rates facilitatesremoval of ammonium sulfate crystals from the screen.

Preferably, the process comprises wiping off ammonium sulfate crystalsfrom the screen with mechanical means, preferably at the side of thescreen to which the feed suspension is fed. This further facilitatesremoval of ammonium sulfate crystals from the screen. Examples ofsuitable mechanical means include scraping means, an agitator, arotating screw. When the abovementioned screening apparatus is used, themechanical means are preferably inside the first chamber. In a preferredembodiment, at least part of the wall of the first chamber forms acylinder, said cylindrical part including at least part of the screen,wherein the mechanical means are inside the first chamber and whereinsaid mechanical means (e.g. scraping means, agitator, screw) can berotated around an axis parallel to the length axis of the cylinder. ARussel Eco Self Cleaning Filter® may advantageously be used.

The size classifying includes transport of ammonium sulfate crystalshaving a sufficiently small size through the openings of the screen.Ammonium sulfate crystals to which the openings of the screen arepermeable, i.e. ammonium sulfate crystals having a sufficiently smallsize that they can permeate through the openings of the screen, can bereferred to as fine crystals and/or as crystals below a predeterminedsize. Ammonium sulfate crystals to which the openings of the screen arenot permeable, i.e. ammonium sulfate crystals having such size that theycannot permeate through the openings of the screen, can be referred toas coarse crystals and/or crystals above the predetermined size. Whenfeeding a feed suspension comprising fine crystals and coarse crystalsto the screen, at least part of the fine crystals are separated from thecoarse crystals as a result of the size classifying.

The dimensions of the openings of the screen are not limited to aspecific value or to any shape. Preferably the openings of the screenhave such dimensions that they are permeable to crystals having adiameter of 0.05 mm, more preferably at least 0.1 mm, in particular atleast 0.2 mm, and more in particular at least 0.5 mm. Preferably, thediameter of the openings of the screen is at least 0.05 mm, morepreferably 0.1 mm, in particular 0.2 mm, and more in particular 0.5 mm.Preferably, the openings of the screen have such dimensions that theyare not permeable to crystals having a diameter of 10 mm, morepreferably 5 mm, most preferably 2 mm.

Preferably, the feed suspension originates from a crystallizer.Preferably, at least part of the permeate suspension is fed to acrystallizer.

Optionally ammonium sulfate solution originating from the permeatesuspension and/or product suspension, e.g. separated by filtration, isintroduced into the feed suspension and/or introduced into the firstchamber, preferably via the feed suspension. This has the advantage thatconcentration ammonium sulfate crystals in the feed suspension isdecreased.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a preferred embodiment of the processaccording to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In this preferred embodiment, use is made of an apparatus (see FIG. 1)comprising an inner tube 1 (the first chamber) and an outer tube 2(second chamber). In the outer tube 2 is an outlet 5. The screen 3 islocated in the wall of the inner tube 1. A narrowing 4 is located at thebottom of the apparatus. The joining point of the inner tube 1 and theouter tube 2 is sealed via a liquid-tight seal. The feed suspension 6enters the inner tube 1 through the top. The suspension 7 then flowsalong the screen 3. Fine ammonium sulfate crystals and ammonium sulfatesolution 8 travel through the openings into outer tube 2 and leaves theouter tube 2 through outlet 5. The stream exiting outlet 5 is thepermeate suspension. The product suspension 9 leaves the inner tube 1through the bottom. The apparatus may be placed in a vertical positionbut this is not necessary.

The invention is illustrated by the following examples without beinglimited thereto.

EXAMPLE I

A feed suspension of 19% by weight ammonium sulfate crystals in anaqueous ammonium sulfate solution, (43% by weight ammonium sulfatedissolved in water) as the continuous phase, was classified using theapparatus shown in FIG. 1. The inner tube 1 was a metal tube of 25 mminside diameter. In the wall of this tube there were located over alength of 20 cm four rows of slits 1.4 mm wide and 5 cm long. The slitswere spaced about 1 mm apart. The longitudinal direction of the slitswas parallel to the longitudinal direction of the inner tube. Anadjustable valve was used for the narrowing at the bottom of the innertube. The outer tube 2 had an inside diameter of about 30 cm. The feedsuspension was supplied through the inner tube from the top at the flowrate of 3 m³ per hour. The streams were controlled so that the flow rateof the product suspension was equal to the flow rate of the permeatesuspension. Samples were taken of both the feed suspension and theproduct suspension. The samples were analysed as follows.

-   1. The sample was filtered with the aid of a “Buchner funnel”.-   2. The crystals obtained were washed with wash liquor consisting of    36.2% by weight methanol, 54.5% by weight water with 9.3% by weight    dissolved ammonium sulfate.-   3. The crystals were washed twice with methanol.-   4. The crystals were washed with diethyl ether.-   5. The crystals were dried at a temperature of 40° C.

Table 1 shows the particle size distribution of the ammonium sulfatecrystals in the feed suspension and the particle size distribution ofthe ammonium sulfate crystals in the product suspension. The amount ofcrystals with diameter <1.25 mm had been reduced by the process of theinvention by 37%.

EXAMPLE II

The procedure described in Example I was repeated. In this instance thefeed suspension contained 4% by weight ammonium sulfate crystals.

The results are shown in Table 1. The amount of crystals with a diameter<1.25 mm had decreased by 52%.

EXAMPLE III

The procedure described in Example I was repeated. The ammonium sulfatesuspension to be classified contained 8.5% by weight ammonium sulfatecrystals. On being rid of solids by filtration, the permeate suspensionwas added to the ammonium sulfate suspension so that a feed suspensioncontaining 4.3% by weight ammonium sulfate crystals was obtained. Theflow rate of the feed suspension was 1.9 m³/h. The streams werecontrolled so that the flow rate of the product suspension was equal tothe flow rate of the permeate suspension.

The results are shown in Table 1. The amount of particles with adiameter <1.25 mm had decreased by 50%.

Examples I to III can be continued with no or only limited occurrence ofclogging of openings in the screen. When examples I to III are repeatedwith the difference that both sides of the screen are not immersed inliquid (as a comparative experiment), the process has to be interrupteddue to the occurrence of clogging and crystallization on the screen.

EXAMPLE IV

A feed suspension of 6.5% by weight ammonium sulfate crystals in anammonium sulfate solution, with 43% by weight dissolved ammonium sulfateas the continuous phase, was classified using the apparatus shown inFIG. 1, but is in this case provided with an agitator, being a screw.The inner tube 1 was a metal tube of 107 mm inside diameter. In the wallof this tube there were located over the total length of 37.2 cm slitsof 1.4 mm wide. The slits were spaced about 1 mm apart. The longitudinaldirection of the slits was parallel to the longitudinal direction of theinner tube. The outer tube 2 had an inside diameter of about 17 cm. Thefeed suspension was supplied through the inner tube from the top at theflow rate of 23 m³ per hour. The streams were controlled so that theflow rate of the product suspension was equal to the flow rate of thepermeate suspension. Samples were taken of both the permeate suspensionand the product suspension. The samples were analysed as follows.

-   1. The sample was filtered with the aid of a “Buchner funnel”.-   2. The crystals obtained were washed with wash liquor consisting of    36.2% by weight methanol, 54.5% by weight water with 9.3% by weight    dissolved ammonium sulfate.-   3. The crystals were washed twice with methanol.-   4. The crystals were dried at a temperature of 40° C.-   5. The particle size distribution of the crystals was determined    with sieve analysis.

Table 2 shows the particle size distribution of the ammonium sulfatecrystals in the permeate suspension and the particle size distributionof the ammonium sulfate crystals in the product suspension. Table 3shows the total concentration of crystals in the feed and in the productsuspension and the mass flow of fines coming with each stream. Theamount of crystals with diameter <1.4 mm had been reduced by the processof the invention by 49%.

EXAMPLE V

The procedure described in Example IV was repeated. In this instance thefeed suspension contained 5.4% by weight ammonium sulfate crystals,while the opening of the slits amounted 0.5 mm. The feed flow amounted21 m³/h. The streams were controlled so that the flow rate of theproduct suspension was equal to the flow rate of the permeatesuspension.

The results are shown in Table 2 and Table 3. The amount of crystalswith a diameter <0.5 mm had decreased by 60%.

EXAMPLE VI

The procedure described in Example IV was repeated. The ammonium sulfatesuspension to be classified contained 26% by weight ammonium sulfatecrystals. The flow rate of the feed suspension was 14 m³/h and the slitopening was 0.5 mm. The streams were controlled so that the flow rate ofthe product suspension was 1.5 times the flow rate of the permeatesuspension.

The results are shown in Table 2 and Table 3. The amount of particleswith a diameter <0.5 mm had decreased by 39%.

TABLE 1 Example II Example III Example III Example I Example I feedsusp. Example II feed susp. product stream feed susp. product susp. % by% by weight product susp. % by % by weight % by weight Particle size, %wt relative to weight relative to relative to total weight relative torelative to total relative to total d (in mm) total solids weight totalsolids weight solids weight total solids weight solids weight solidsweight d > 3.35 6.82 6.09 2.19 4.06 3.6 4.7  2.0 < d < 3.35 61.96 65.4348.19 57.4 20.6 26.4  1.7 < d < 2.0 17.04 17.05 22.26 21.05 19.2 24.4 1.4 < d < 1.7 7.25 6.72 11.62 9.24 21.5 25.2 1.25 < d < 1.4 2.3 1.834.27 2.76 7 5.3  0.8 < d < 1.25 3.28 2.12 7.19 3.69 18.3 9.9  0.4 < d <0.8 1.18 0.64 3.63 1.55 8.7 3.7 d < 0.4 0.17 0.12 0.65 0.25 1.1 0.4 d <1.25 mm 4.6 2.9 11.5 5.5 28.1 14.0 Example I Example I Example IIExample II Example III Example III feed suspension product suspensionfeed suspension product suspension feed suspension product suspension %by 19 40 4 8 4 7 weight solids relative to sum of liquid + solids

TABLE 2 Example V Example VI Example VI Example IV Example IV permeatesusp. Example V permeate susp. product stream permeate susp. % productsusp. % by weight product susp. % by % by weight % by weight particlesize, by weight relative to % wt relative to relative to total weightrelative to relative to total relative to total d (in mm) total solidsweight total solids weight solids weight total solids weight solidsweight solids weight d > 1.7 0.72 13.69 0 0.28 0.01 17.77 d < 1.7 99.2886.31 100 99.72 99.99 82.23 d < 1.12 59.55 38.35 99.6 62.49 99.93 55.2 d< 0.8 34.52 20.67 97.89 33.83 99.34 38.07 d < 0.6 20.53 12.52 85.1519.03 91.18 25.56 d < 0.425 11.39 7.3 50.4 9.76 60.95 13.75 d < 0.2 2.81.82 11.97 2.35 18.06 2.83

TABLE 3 Example IV Example IV Example V Example V Example VI Example VIfeed suspension product suspension feed suspension product suspensionfeed suspension product suspension % by weight 6.5 8.2 5.4 7.4 26 38solids relative to sum of liquid + solids Mass flow of 1642 844 343 1381386 846 fines kg/h

1. Process for size classifying ammonium sulfate crystals, said processcomprising the steps of: (a) providing a screen having screen openingswithin a vessel so as to establish first and second volumes within thevessel on respective first and second sides of the screen; (b) feedingto the first side of the screen an ammonium sulfate feed suspensioncomprised of an ammonium sulfate solution containing ammonium sulfatecrystals, the ammonium sulfate crystals comprised of coarse ammoniumsulfate crystals having diameters sufficiently large to prevent passagethrough the openings of the screen and fine ammonium sulfate crystalshaving diameters sufficiently small to allow passage through theopenings of the screen; and (c) while simultaneously maintaining thefirst and second sides of the screen immersed in contact with theammonium sulfate solution, size classifying the ammonium sulfatecrystals based on crystal diameters by allowing a substantial part ofthe fine crystals to pass through the openings of the screen to thesecond volume of ammonium sulfate solution within the vessel on thesecond side of the screen while the coarse ammonium sulfate crystals areretained in the first volume of ammonium sulfate solution within thevessel on the first side of the screen.
 2. Process according to claim 1,further comprising: (d) withdrawing a portion of the second volume ofammonium sulfate solution as a permeate suspension comprised of the fineammonium sulfate crystals in an ammonium sulfate solution from thesecond side of the screen, and (e) withdrawing a portion of the firstvolume of ammonium sulfate solution as a product suspension comprised ofthe coarse ammonium sulfate crystals in an ammonium sulfate solutionfrom the first side of the screen.
 3. Process according to claim 2,comprising: providing a screening apparatus comprising a first chamberfor holding the first volume of ammonium sulfate solution, a secondchamber for holding the second volume of ammonium sulfate solution, andthe screen, the screen forming a partition between the first chamber andthe second chamber, and wherein the process comprises: introducing thefeed suspension into the first chamber, withdrawing the permeatesuspension from the second chamber, and withdrawing the productsuspension from the first chamber.
 4. Process according to claim 3,wherein the process comprises controlling the flow rate of the feedsuspension fed to the first side of the screen, the flow rate of thepermeate suspension withdrawn from the second volume of ammonium sulfatesolution and/or the flow rate of the product suspension withdrawn fromthe first volume of ammonium sulfate solution, while simultaneouslymaintaining the first and second sides of the screen immersed in contactwith the ammonium sulfate solution.
 5. Process according to claim 1,wherein the feed suspension which is fed to the screen comprises lessthan 25 vol.% ammonium sulfate crystals.
 6. Process according to claim2, wherein the product suspension which is withdrawn from the screencomprises less than 50 vol.% ammonium sulfate crystals.
 7. Processaccording to claim 1, wherein transport of the suspension to the firstside of the screen is in a direction essentially parallel to the screen.8. Process according to claim 7, wherein transport of the suspension tothe first side of the screen is at a rate of at least 0.01 m/s in adirection parallel to the screen.
 9. Process according to claim 1,wherein the process comprises wiping off ammonium sulfate crystals fromthe first side of the screen with a mechanical wiper.
 10. Processaccording to claim 9, comprising providing a screening apparatuscomprising a first chamber for holding the first volume of ammoniumsulfate solution, a second chamber for holding the second volume ofammonium sulfate solution and the screen, the screen forming a partitionbetween the first chamber and the second chamber, and wherein themechanical wiper is inside the first chamber.
 11. Process according toclaim 10, wherein at least part of the wall of the first chamber forms acylinder which includes, at least part of the screen, wherein themechanical wiper is inside the first chamber and rotatable around anaxis parallel to a length axis of the cylinder.
 12. Process according toclaim 1, wherein step (a) is practiced so as to prevent contact of theopenings of the screen with air.
 13. Process according to claim 3,wherein the permeate suspension is withdrawn from the second chamber atan end thereof which is proximate to an end of the first chamber intowhich the feed suspension is introduced and wherein the productsuspension is withdrawn from the first chamber at an end thereof remotefrom the end into which the feed suspension is introduced.
 14. Processaccording to claim 1, wherein the openings of the screen comprise slits.15. Process according to claim 14, wherein the feed suspension isintroduced into an inner tube which comprises the screen, and whereinthe slits have a longitudinal dimension which is oriented parallel to alongitudinal direction of the inner tube.
 16. Process according to claim1, wherein the feed suspension which is fed to the screen comprises lessthan 50 vol.% ammonium sulfate crystals.
 17. Process according to claim2, wherein the product suspension which is withdrawn from the screencomprises less than 70 vol.% ammonium sulfate crystals.